Fig. 3

Example where the minimal edit path requires contracting a good edge: if we contract the internal good edge of T (the bold one), then the 3 subtrees of T can be handled together, requiring 6 node flips and 18 edge contractions to reduce T into a star tree, and then 18 edge extensions to reach T^′, leading to 42 operations in total. By contrast, if we do not contract the good edge of T, then the two subtrees of T separated by this edge should be handled separately, requiring 9 flips, 17 edge contractions and 17 edge extensions to reach T^′, leading to 43 operations in total. The first scenario is the better one